Originally Posted by Prateek M
We used two photoelectric sensors on our intake to detect if a cube was in or not. When picking up the second and third cubes for autonomous, we only continued with the routine when the sensors said the cube was in. This can be seen in our match videos (the LEDs on the robot blink when the sensors detect a cube -- this also helped the driver know that he had a cube when the robot was on the other side.)
A picture of the intake with the two sensors can be found here: http://prntscr.com/leqdsh
These seem really nice because they look easier to mount than the IR sensor mentioned by Justin, while accomplishing the same non-contact sensing. Though I bet they are more expensive. Would you be able to share where you bought them?